Add flake.nix

[pingiun]

Fixes #61

• Tests pass
• Appropriate changes to README are included in PR

[pingiun]

paging @colemickens and @arxanas for review

[arxanas]

Can you add a CI job to verify the Nix build? Example here: https://github.com/arxanas/git-branchless/blob/master/.github/workflows/nix-linux.yml. (git-branchless would break the Nix build all the time if not for that.)

[martinvonz]

Fixes #61

Thanks!

• Appropriate changes to README are included in PR

I guess this is about adding a note to the installation instructions? Makes sense.

Anything else I should wait for before merging the PR? Were @arxanas and @colemickens happy with the review? (Sorry if that's indicated somewhere. I'm new to collaborating on GitHub, and especially the mobile UI.)

[arxanas]

LGTM assuming CI passes.

[colemickens]

flake.nix looks good, I think my only feedback would be to try to patch the version so that this is more accurate:

❯ nix run github:martinvonz/jj/9fb580b6eeba2bbd79658f9777000cabb2ebdaed -- --version
jj 0.2.0 

Ideally that might look like:

❯ nix run github:martinvonz/jj/9fb580b6eeba2bbd79658f9777000cabb2ebdaed -- --version
jj unstable-9fb580b6

or something like that. Not a blocker though! Love to see it, Nix ftw!